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Additionally, the performance of the watch face should be optimized.
Services must not perform unnecessary computations. Watch faces with
animations must run smoothly while accommodating
notifications and system indicators.

Basic optimization

This section contains best practices for improving efficiency.

Color and brightness of a watch face

Dark colors conserve the battery of a watch.
The following are recommendations for setting the watch face background,
to optimize the watch face's battery use:

Color. Whenever possible, use a black background.

Brightness. When a watch face's requirements prevent the
use of a black background, you should strive to keep the brightness of
the background color at or below 25 percent on an
HSV (Hue, Saturation, Value) or HSB scale. For example, if you use the
Color
class to set the background color, and define the color with the HSV scale,
you would use 25 or less for the Value setting (that is, for the
brightness setting).

Use callbacks in WatchFaceService.Engine

Ensure that your watch face performs
computations only when active; use callbacks
in WatchFaceService.Engine.
Preferably, use the following methods of that class to determine if
the watch face is visible:

Do not register a broadcast receiver in the Android manifest file
to get system events such as time zone changes, battery events, etc., because
the BroadcastReceiver
is called whether or not a watch face is active. However, you can use the
registerReceiver method
of the Context class to register a receiver.

Monitor power consumption

The
Android Wear companion app enables developers and users to see how much battery
is consumed by different processes
on the wearable device (under Settings > Watch
battery).

For information about features introduced in Android 5.0 that help you improve battery life,
see Project Volta.

Register encryption-aware watch faces

Android 7.0 and higher include support for file-based encryption and allows
encryption-aware
applications
to run before the user has provided the decryption passcode at bootup. This can reduce the
duration of transitioning from boot animation to the watch face by up to 30 seconds.

Note: Use this feature with watch
faces that do not use credential encrypted storage.

Best practices for animations

The best practices in this section help to reduce the power consumption of animations.

Reduce the frame rate of animations

Animations are often computationally expensive and consume a significant amount of power. Most
animations look fluid at 30 frames per second, so you should avoid running your animations
at a higher frame rate.

Let the CPU sleep between animations

Animations and small changes to the contents of the watch face wake up the CPU. Your watch
face should let the CPU sleep in between animations. For example, you can use short bursts of
animation every second in interactive mode and then let the CPU sleep until the next second.
Letting the CPU sleep often, even briefly, can significantly reduce power consumption.

To maximize battery life, use animations sparingly. Even a blinking colon wakes up the CPU with
every blink and hurts battery life.

Reduce the size of your bitmap assets

Many watch faces consist of a background image and other graphic assets that are transformed
and overlapped on top of the background image, such as clock hands and other elements of the design
that move over time. Typically these graphic elements are rotated (and sometimes scaled) inside the
Engine.onDraw()
method every time the system redraws the watch face, as described in
Draw Your Watch
Face.

The larger these graphic assets are, the more computationally expensive it is to transform them.
Transforming large graphic assets in the
Engine.onDraw()
method drastically reduces the frame rate at which the system can run your animations.

Figure 1. Clock hands can be cropped to remove extra pixels.

To improve the performance of your watch face:

Do not use graphic elements that are larger than you need.

Remove extra transparent pixels around the edges.

The example clock hand on the left side of Figure 1 can be reduced in size
by 97%.

Reducing the size of your bitmap assets as described in this section not only improves
the performance of your animations, but it also saves power.

Combine bitmap assets

If you have bitmaps that are often drawn together, consider combining them into the same
graphic asset. You can often combine the background image in interactive mode with the tick
marks to avoid drawing two full-screen bitmaps every time the system redraws the watch face.

Disable anti-aliasing when drawing scaled bitmaps

When you draw a scaled bitmap on the Canvas object using the Canvas.drawBitmap() method, you can provide a Paint instance to configure
several options. To improve performance, disable anti-aliasing using the setAntiAlias() method, since this option does not have any
effect on bitmaps.

Use bitmap filtering

For bitmap assets that you draw on top of other elements, enable bitmap filtering on the same
Paint instance using the setFilterBitmap() method. Figure 2 shows a magnified view of a clock hand with
and without bitmap filtering.

Note: In low-bit ambient mode, the system does not reliably
render the colors in the image for bitmap filtering to process successfully. When ambient mode is
active, disable bitmap filtering.

Move expensive operations outside the drawing method

The system calls the
Engine.onDraw()
method every time it redraws your watch face, so you should only include operations that are
strictly required to update the watch face inside this method to improve performance.

When possible, avoid performing these operations inside the
Engine.onDraw()
method:

To analyze the performance of your watch face, use the Android Device Monitor. In particular,
ensure that the execution time for your
Engine.onDraw()
implementation is short and
consistent across invocations. For more information, see
Inspect Threads and Method Traces.